The invention has general application to the construction of hydraulic cylinders. However, a particular application relates to large pull-down presses. Such presses generally have a frame, a lower platen fixed to the frame, and an upper displaceable platen. Two or more hydraulic cylinders may displace the upper platen between an upper retracted position in which work pieces can be removed from or placed between the platens and a lower operative position in which a work piece can be compressed between the platens. Each hydraulic cylinder contains a piston that divides the interior of its housing into upper and lower chambers. One chamber discharges hydraulic fluid to a reservoir whenever the other chamber expands under pressurized fluid flows from a pump. During actual compression phases of operation, very little hydraulic fluid may flow to or from the chambers. However, during rapid advancing and retraction of the upper platen to and from its lower operative position, flow rates approaching 5000 liters per minute would not be uncommon in large-scale presses, if reasonable press cycle times are to be maintained. Conduits and flow valves must be appropriately sized to accommodate such flow rates.
Techniques are known for more direct transfer of hydraulic fluid between cylinder chambers and are commonly referred to as "regeneration". Regeneration has generally required a flow circuit external to a hydraulic cylinder and external flow gating valves. A recent proposal suggests that the regenerative flow path and valves may be formed within the cylinder itself. The cylinder rod is formed with a hollow interior that communicates through ports with the chamber at the rod end of the cylinder. A hollow tube is fixed to the base or blind end of the cylinder and extends in telescopic relationship through the cylinder piston and the interior of the cylinder rod. A short passage formed in the base of the cylinder places the hollow tube in communication with the lower chamber. The rod, tube, and passage together define a complete internal flow path between the upper and lower chambers. A valve mounted in the base of the cylinder, specifically in the short flow passage in the base, controls fluid flows. The valve is configured to open in response to pressure within the flow path, allowing direct flows between cylinder chambers. Hydraulic pressure along a pilot line through the base of the cylinder closes the valve, isolating the chambers. Regeneration can be achieved both during extension and contraction of the cylinder. However, the telescopic mounting of the hollow tube introduces considerable complexity, and necessarily reduces the effective cross-sectional piston area responsive to pressure during cylinder extension.